Agricultural irrigation machines of the type known as center pivot machines have a main pipeline section supported at intervals on movable towers for rotation about an inner end of the pipeline. Water is supplied to the fixed, inner end of the pipeline and distributed through sprinklers or other fluid emitting devices placed along or supported from the pipeline. The movement of the main pipeline section about its central pivot point irrigates a circular portion of a field. The circular portion swept by the main pipeline is designated herein the primary field space. This leaves the corners of square or rectangular fields or other irregular perimeter areas without irrigation. In installations where the additional productive capacity of a field's corners or irregular perimeter areas warrants, the corners or irregular perimeter areas can be irrigated by adding an auxiliary span or spans near the outer end of the main pipeline for irrigating areas of the field outside of the circular area. These areas to be irrigated outside the primary field space are herein designated the secondary field space. The auxiliary span or spans are in fluid communication with and supplied by the outer end of the main pipeline section. Such an auxiliary structure is capable of moving as needed to extend into the corners or other irregular areas while also moving to keep up with the main pipeline section's rotation about the center pivot.
A commonly-used motion control scheme for center pivot units causes the outermost tower on the main pipeline section to advance independently at a user-defined speed. All inboard towers advance as needed to keep the main pipeline straight. The auxiliary structure may be pivotally connected near the outer end of the main pipeline and supported on an auxiliary tower. The auxiliary tower may travel inside or outside of the main circle, following its own guide path. That path may be defined by various means, such as a buried cable. The guide path for the auxiliary tower extends at least partially into the corners or irregular perimeter areas of the fields, thereby causing the auxiliary structure to move out into any irregular area when the main pipeline is adjacent to such a feature. As the auxiliary structure moves into, through, and out of such secondary field space, the fluid emitting devices mounted thereon or supported therefrom are required to provide varying amounts of fluid as the field surface “area of responsibility” for each emitting device changes and as the velocity of each emitting device changes. When the main pipeline is adjacent the side of a field the auxiliary tower usually lags behind the main pipeline, thereby folding the auxiliary structure back in to a trailing position behind the main structure. With the auxiliary structure in this position the fluid emitting devices mounted thereon are required to provide very little if any contribution as the field surface area is effectively treated by the emitter devices on the main pipeline.
The object of both the main pipeline span and the auxiliary span is to apply to the field a user-specified amount of fluid, typically expressed in inches, both in the primary field space and in the secondary field space to suitably treat the surface area affected by each of the individual fluid application devices thereon. Since the emitters on the main pipeline move in a relatively uniform way, adjustments in their size and spacing can readily accommodate their differences in area of responsibility arising from different radial locations on the main pipeline. However, such adjustments cannot be used to address the situation faced by the auxiliary span emitters. The movements of these emitters create irregularities in both the sizes of their zones of responsibility and the amount of time they spend in those zones. These irregularities make it difficult to apply the desired amount of irrigation in the secondary field spaces.
In the past, controls for the emitters on the auxiliary span have relied primarily on a measurement of a control angle between the auxiliary span and the main pipeline. Different auxiliary span emitters, typically ganged into groups or banks of emitters, were successively turned on as the control angle increased beyond 85°. Eventually when the control angle became large enough, which meant the auxiliary span was well extended beyond the primary field space, all of the banks were turned on. As the auxiliary span folded back in and the control angle became smaller, the banks were successively turned off. The banks were set up such that a particular bank did not include any adjacent emitters. For example, every tenth emitter would be in a particular bank. Thus, the banks were fully interleaved.
This prior art auxiliary emitter control method for the most part is able to prevent auxiliary emitters from irrigating the primary field space, which would otherwise over-irrigate the outer portions thereof. And the prior art control method generally assures that most of the secondary field space will receive water in some amount. That amount may be too much, it may be too little, or perhaps it may even be the desired amount. Whatever the case may be, the prior art control method can not uniformly vary the average amount of water which is emitted. At most it can turn an emitter on and achieve its full flow rate for a sustained period of time, or turn it off and have a zero flow rate.
As mentioned above, the problem is that both the size of the area of responsibility of a particular emitter and the time available to that emitter in which it can apply water to that area are constantly changing. Furthermore, both of these parameters vary radically among the various emitters on the auxiliary span. The prior art control methods and apparatus are incapable of adequately dealing with these variations.